Fractionation of gaseous mixtures



Jall- 1, 1952 LQD. ETHERINGTQN ETAL 2,581,088

FRACTIONTION OF GASEOUS MIXTURES Filed Nov. 3, 1947 flvenbors @SMMCm/oraefj Lewis 2D; E Ha fold CJ.'

dmJOOU Passed nn. '1,' 4195i 2,581,033y

"UNITED sTATEs PATENT osi-ICE 2,581,088 v FRACTIONATION F GASEOUS MIXTURES Lewis D. Etherlngton, Bayonne, and Harold W. Scheeline, West Orange, N. J., assixnors to Standard Oil Development Company, a corporation of Delaware Application November 3, 1947, Serial No. 783.834 3 Claims. y(Cl. 62-2) {Ll-,This invention relates'tov the separation and f ular weight hydrocarbons from admixture one purificationof constituents of normally gaseous with the other, the total reflux refrigeration has mixtures, and in particular to the recovery of been applied at the uppermost or coldest part of ethylene and propylene from hydrocarbon mixthe column. The point in such a fractional distures. 6 tlllation column where the maximum reflux re- The separation of normally gaseous constitquirement (which fixes the total refrigeration uents from complex mixtures has become, espeload) normally occurs is at about the introduccially in the case of air fractionation, a highly tion of the feed, that is. the point at which a developed industrial practice. For some time, relatively high temperature exists. The minithe importance of the low molecular weight hy- 10 mum reflux requirement on the other hand, in

drocarbons, as chemical intermediates, particuthe system is at Vthe top of the column. In the larly ethylene and propylene, has necessitated f case in which the total refrigeration is supplied consideration of the application of similar techat the coldest point in the column, the minimum niques for the fractionation of low boiling hydronumber of plates is required but the load upon carbon mixtures. For the separation of low mol5 the refrigeration system is heavy, the refrigeralecular weight hydrocarbons from the abundantly tion power requirement and cost being inversely available mixtures at oil fields and petroleum proportional to the temperature level.

refineries, the employment of similar operating In the present invention, the refrigeration of procedures to those used in air fractionation a fractionating system is effected in stages, that has been previously proposed. In 'the present 20 is, only part of the total rei'rigeration requireinvention the separation of these low boiling ment is effected at the lowest temperature level hydrocarbons is achieved by distillation in which while the remainder of the cooling required above by reason of a numberof controlled heat exthe point of feed supply is effected throughout change stages for removing or adding heat to the distillation zones at various stages in the the distillation system a high order of thermofractionation and at progressively increasing temdynamic eiliciency is attained. The novel operperature levels. With 'the heat removal from the ative factors involved in this type of distillation fractionation zone by the extraneously controlled are also features of this invention. refrigeration means above the point of feed sup- It has generally been found that the employply eilected in this manner, the average temperament of rectifying columns is the best means for ture level of the refrigeration source may be separating low molecular weight hydrocarbons raised. Thus, while the number of separation from their admixture one with the other. In stages (i. e., plates or height of packing) in the a iractionating column, the feed mixture, as a tower will have to be increased, the total power gas or partially liquied system, is supplied at a requirement involved in the refrigeration sysrelatively cold point, about the center of the col- 3i tem will be greatly reduced and in most cases umn, the more volatile constituents are sepaan overall economy will result. External cooling rated as vapor and the heavier constituents pass means, such as expansion engines and similar to the lower portion of the column as liquid. devices, may be employed in product stream lines Usually, pressure of the order oi between 200 at various stages to supply added cooling by and 500 pounds per square inch is applied to the 40 recycling such streams or as additional refrig-` column in order to raise the temperature level eration sources. 4 and to make more compact the fractionating Also, in the prior art, in a fractional distilsystem. In applying pressure to a hydrocarbon lation column operated for the concentration and mixture, the difference in relative volatilitieskderecovery of low molecular weight hydrocarbons creases and thus the need exists for more exten- 4 from admixture one with the other, heat has sive reflux. `Also, considerable energy must be been applied totally at the bottom or hottest part f expended in compressing the gaseous feed mixot' the column although an appreciable portion of ture. The present invention is an improvement the total heat requirement could be applied at I over such processing in the employment of presa point just below the point of feed entry where sures of about the order of those of the feed a much lower temperature level exists. In the supply as normally obtainable in a petroleum recases in which the heat at the bottom of the nely. column was required at a comparativelylow- In the prior art, in a fractional distillation coltemperature level, the heat has been supplied umn operated at sub-atmospheric temperatures in the prior art by the condensation of refrigerior the concentration and recovery of low molec- 5 ant vapor. thus providing inexpensive by-product refrigeration for some higher temperature operation. In such a case where low-temperature heat is applied totally at the bottom of a fractionation column, the minimum number of plates or separation stages is required but the refrigeration available from the heating is at the highest or least valuable temperature level.

In the present invention, the heating for a lowtemperature fractionating system is effected in stages; that is, only part of the heat is supplied at the highest temperature level while the remainder of the heating is effected in various stages throughout the zone between the bottom and feed entry points of the column and at progressively-decreasing temperature levels. Thus, while the number of separation stages in this zone or the tower will have to be increased, byproduct refrigeration may be supplied at a much lower average temperature level.

In general, therefore, the present invention relates to the fractional distillation oi' low molecular weight normallygaseous hydrocarbons, that is, to the separation mainly of the C1, Cz, and C: hydrocarbons from their admixture with each other and minor amounts of the C4 and C5 hydrocarbons by passing the hydrocarbon mixtures into a distillation zone having therein at least three extraneously controlled heat exchange means at different temperature levels, thus per- .mitting very close separation of the constituents of the mixtures in a particularly advantageous manner from an economic point of view. Expansion engines and similar cooling means may also be used for cooling product streams to provide added refrigeration especially at the lowest operating temperature levels.

The process of the invention as applied to the separation of the constituents of mixtures such as are currently available in a petroleum refinery, as cracked stocks and still gases, eiects the separation between hydrocarbons having substantial differences in boiling points. The fractionation of the various mixtures is generally effected at pressures about the same as those at which the mixtures are obtained from the producing units. To illustrate th'm variation, the boiling points of the common constituents of such mixtures are at l atmosphere pressure:

C. Hydrogen 252.54 Methane 161.6 Ethylene 103.0 Ethane 88.'1r Propylene 47.0 Propane 42.2 Isobutane 12.2 Isobutylene 6.6 Normal butylene 6.1 Normal butane 0.6 Beta-butylene +9.5

Tertiary pentane +2785 Secondary pentane +2195 Normal pentane +3606 In order that the invention may be more fully appreciated and understood, the following description of an embodiment is presented. In this connection, a typical flow diagram for processing is presented in the drawing. In the drawing is shown feed supply entering through line I0. In this particular embodiment, the'feed supply at about 110 F. and 30 pounds gauge pressure is obtained from a propane cracking unit. A typical analysis of a feed supply is as follows:

. exemplified above Approx. mol. per cent Hydrogen (Hz) 10 Methane (CHA) 30 Ethylene (02H4) 30 Ethane (02H5)- 6 Propane (Cal-Ia) A l Propylene (CsH) 14 Butylenes 8 Amylene, pentane, benzol and toluol 1 Such a mixture is first passed through drying equipment` I I. The drying agent is of any of the commonly available types well known in the lndustry, such as activated alumina. The mixture is then cooled in equipment I2 by heat exchange with cold product streams and with ammonia refrigerant to a temperature of about 33 F. The product streams may have been passed through expansion devices to obtain low prevail ing temperatures. At this temperature and at a pressure of about 25 pounds gauge, mixtures as are at about their liquefaction point. The particular mixture is then passed to tower I3. The tower I3 is fitted with one or more lower heating coils Il, one or more upper cool.- ing coils I5, and has about tower I3, the C4 and heavier hydrocarbons are separated as bottoms at about F. from the more volatile constituents as vapor at about 66 F. The bottoms are removed through line I6 to be further treated as desired. Reux is provided by contact of the upwardly flowing vapors with the cooling coils I5' through which ethylene under pressure as a refrigerant at 80 F. is passed. The overhead vapors passing through line l1, consisting largely of C3 and lower molecular weight hydrocarbons, pass to tower I8 wherein ethane and lighter hydrocarbons are separated from propylene and propane. The tower I3 has about 30 plates and is fitted with coils I3, 2l and 2l. Coil I9 is usually a heating coil containing condensing high pressure ethylene, while 22 contains ethylene as a refrigerant at about F., and coil 2I contains methane as a refrigerant at about 183 F. The tower i8 is operated at about 25 pounds gauge pressure.

The overhead product from tower I8, at a temperature of about 137 F., passes from the tower through line 22 to tower 23. The bottoms from the tower I8, at a temperature of about 25 F., pass through the pump 2l and thence through heat exchanger I2 for storage as desired. If the C: product is desired in vapor form its heat of vaporization can also be utilized to cool the feed stream and thus effect a saving of ammonia refrigerant. The tower 23 contains about 20 plates and is fitted with cooling coils 25 and 26. 'In this tower the highly volatile constituents, that is. mainly methane and hydrogen are separated from ethylene and ethane. The tower 23 is cooled by methane refrigerant boiling in coils 25 and 26 at temperatures of about 183 F. and 218 F.. respectively. Also a small quantity of low pressure liquid methane is supplied as reflux through line 21 near the top of the tower. The overhead vapors passing through line 23 are largely hydrogen and methane. These vapors may be passed directly to the heat exchanger I2 or through expansion engine 39. cooler 40 and then through one of the coils I5 in column I3.

The distillation residue, consisting of C: constituents and some dissolved methane, is then passed through line 29 to the tower 30. The tower 30 contains about l0 plates, the heating coil 3l which contains condensing methane or condensing tower I3 vapor, and operates at about 20 plates. In the pounds gauge pressure. The distillation residue from the tower 23 passing through line 29 therefore undergoes a pressure reduction in an expansion engine or valve 58 and therefore a temperature reduction before entering the tower 30. Into the top of the tower 30 through line 3| is supplied low pressure methane reflux. From the tower 30, the overhead vapors consisting mainly of methane pass through line 32 to compressors 36 and cooler 37 to be employed subsequently for refrigeration purposes. In compressing the methane overhead, the amount is gauged to be equivalent to the reflux to be supplied to the towers 23 and 30 together with the methane losses from the refrigeration system. Ethylene of about 74% purity (balance, largely ethane) is obtained as distillation residue from the tower 30 as liquid and is compressed in pump 35 to about 400 pounds gauge before passing to heat exchanger I2. If the cooling capacity of the C2 product stream is greater than the cooling required for the feed stream, the C2 stream can be utilized for providing reflux to one of the distillation columns, such as either column I3 or column I8.

For towers I8 and 30 which require the addition of low temperature heat, such heat may be supplied by the cooling of the original feed mixture or by the condensation of vapors from the upper portions of the various distillation zones. In this manner the condensed vapors may be utilized as reflux to the distillation zones and the saving in overall refrigeration is eiected. The heat to towers I8 and 30 is shown as being supplied and however, to employ a in towers I8 and 30 which are located at various stages between the bottom and feed points. By this alternate method, the heat is supplied at progressively decreasing temperature levels and hence lower temperature refrigeration will be available for providing reflux to the appropriate towers or for cooling the original mixture.

The heating and cooling in the various distillation zones may be effected by means of coils.

fins or other such heat exchange means located within or without the distillation zones provided that the temperature of the various heat exchange media are controlled extraneously to the respective distillation zone. The contact between vapor and liquid within the fractional distillation zones may be aided by means of bubble cap plates, perforated plates, packing, or dephlegmator tubes.

What is claimed is:

1. A method of supplying the heat requirements in the fractional distillation of a mixture of normally gaseous hydrocarbons which comprises passing the said normally gaseous hydrocarbons into a distillation zone having at least two extraneously controlled heating media below the point of feed inlet maintained at different temperature levels.

2. In the operation of a fractional distillation tower characterized by successively lower temperatures from the bottom to the top of the tower the improvement which consists of maintaining a plurality of heating media in the said tower below the point of feed inlet maintained at successively lower temperature levels.

3. In a fractional distillation operation the method of increasing the thermodynamic eiciency of heating the portion of the feed stream moving downwardly in the fractionation tower in liquid phase which comprises successively contacting the said portion of the feed stream with a plurality of heating media maintained at successively higher temperatures with respect to the movement of the liquid portion of the feed stream.

LEWIS D. ETHERINGTON.

HAROLD W. SCHEELINE.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Aug. 8, 1944 

1. A METHOD OF SUPPLYING THE HEAT REQUIREMENTS IN THE FRACTIONAL DISTILLATION OF A MIXTURE OF NORMALLY GASEOUS HYDROCARBONS WHICH COMPRISES PASSING THE SAID NORMALLY GASEOUS HYDROCARBONS INTO A DISTILLATION ZONE HAVING AT LEAST TWO EXTRANEOUSLY CONTROLLED HEATING MEDIA BELOW THE POINT OF FEED INLET MAINTAINED AT DIFFERENT TEMPERATURE LEVELS. 